None.
Not Applicable.
1. Field of the Invention
This invention relates generally to motor vehicle wheel balancing systems, and more particularly to a runout measuring device for improving the mounting of a vehicle tire/wheel assembly on a vehicle hub.
2. Related Art
A motor vehicle""s ride quality is greatly influenced by the balance of the wheels it rides on. The balancing of vehicle wheel assemblies is most often accomplished by removing the tire/wheel assemblies from the vehicle and mounting each of the tire/wheel assemblies on an off-car-balancer. The off-car balancer rotates the tire/wheel assembly, measures the imbalance forces and displays the amount and location of weight to add to the wheel to bring the tire/wheel assembly into a balanced condition. International Standard ISO 1925 (1990) by International Organization for Standardization, incorporated by reference herein, defines balancing terms such as single-plane balance, two-plane balance, couple unbalance, dynamic unbalance, residual unbalance, and principal inertia axis.
Some off-car balancers, such as the Hunter GSP 9700 Vibration Control System, also measure lateral runout and radial runout of the rim and loaded runout of the tire""s outer diameter to help match the mounting of the tire to its rim. Runout is the amount that a surface""s trajectory, runs out of a true circle centered about the axis of rotation. Lateral runout is measured parallel to the axis of rotation, and radial runout is measured perpendicular to the axis of rotation.
The runout measurements can be expressed using a variety of mathematical terms. The first harmonic, once per revolution, discrete Fourier transform is one term that can be used to express runout. Another term that can be used is total indicator reading (TIR), which is the maximum deviation of any point on the surface from a true circle minus the minimum deviation of any point on the surface from the true circle. As described in U.S. Pat. No. 5,103,595, the least squares best fit method can also be used to express runout. The high spot of runout or the location of maximum runout is the angular location about the axis of rotation at which the mathematical term being used to express runout is at its maximum value.
The runout of the outer surface of a tire measured using a typical tire load is often quite different than unloaded runout, the runout measured without load. This is caused by circumferential variations of tire stiffness and the tire deflection when supporting the weight of a vehicle. Off-car balancers with runout measuring capability are useful in matching a tire to its respective rim, especially when the rim is designed with some runout to offset runout in the tire. The high spot of loaded runout of the tire is matched with the low spot of the rim to obtain an assembly with minimum loaded runout. A tire/wheel assembly must have a low level of loaded runout as well as a low level of imbalance in order to operate free of vibration on the vehicle.
Loaded radial runout of a tire is a useful measurement of tire uniformity that corresponds to the amount of vibration that the tire will impart to the vehicle. Tire uniformity measuring machines can quantify the force variation of a tire and are often used by tire manufacturers to inspect tires. The Society of Automotive Engineers Recommended Practice J332 (August 1981), included by reference herein, describes the design requirements for such measuring machines and defines force variation. Loaded radial runout and force variation are closely related. The high spot of force variation is usually the high spot of loaded radial runout, and loaded radial runout multiplied by the stiffness of a tire is a good approximation of the force variation of a tire. Additionally, the angular location of maximum radial force variation is substantially the same as the location of maximum radial loaded runout.
With the use of computers, modem vehicle wheel balancers are capable of imbalance measurements repeatable to within a few hundredths of an ounce when a vehicle tire/wheel assembly remains mounted on the balancer between measurements. The repeatability of the measurements is reduced when the tire/wheel assembly is removed from the balancer and remounted between spins. A primary cause for loss in repeatability is inaccurate mounting of the tire/wheel assembly on the balancer. Errors can be introduced by clearance between the cone and the balancer shaft, runout in the balancer cone or shaft, runout in the hub face, and imbalance in the balancer.
Although a tire/wheel assembly may be balanced so that it produces negligible forces when rotated on the off-car balancer, the same assembly may cause significant imbalance forces when mounted on the vehicle and rotated using the vehicle""s bearings and axle. The imbalance forces of a tire/wheel assembly will remain constant between the off-car balancer and the vehicle only if the relationship between the tire/wheel assembly and the axis of rotation is the same for the two mountings. Achieving the desired on-car wheel balance with only an off-car balancer involves both accurately mounting the wheel on the balancer and then accurately mounting the tire/wheel assembly on the vehicle""s hub. Possible causes of wheel to vehicle mounting inaccuracy include clearance between the balancer hub and the rim pilot hole, runout of the hub pilot diameter or mounting face, rust or grime between rim and vehicle hub, runout in studs and runout in lug nuts.
On-car balancers can eliminate some of the mounting accuracy problems by performing the balance measurements after the tire/wheel assembly is in its final mounted position on the vehicle. Although on-car balancers are available, they are not very popular because of setup difficulties, operational limitations, and safety issues. To install an on-car balancer, an instrumented jack stand must be mounted under the lower suspension member at each tire/wheel assembly. For best accuracy, the calibration of on-car balancers must be adjusted for each vehicle. It is also important that the jack stand is mounted securely on the suspension next to the tire/wheel assembly because the assembly must be spun at a high speed. However, operators generally find it difficult to ensure that the jack stand is properly mounted, causing safety concerns. Additionally, on-car balancers only output the amount and angle of a single weight to attach to the wheel, and they cannot compute the best distribution of the correction weight between the inner and outer rim flanges. On-car balancers are also called finish balancers because an off-car two-plane balance is typically performed on a tire/wheel assembly before it is mounted to the vehicle, and then the on-car static balance is performed.
It is in view of the above problems that the present invention was developed. The invention described herein preferably includes an off-car balancer, an on-car runout device, and a computer that receives information from the off-car balancer and the on-car runout device. The off-car balancer preferably includes a runout measuring device and a tire uniformity measuring device that respectively measure the runout of two surfaces of the tire-wheel assembly and measure the loaded radial runout, or radial force variation, of the tire/wheel assembly. The on-car runout device measures the vehicle hub to determine the radial runout of the wheel mounting studs, the radial runout of the hub pilot diameter and the lateral runout of the hub face. The on-car runout device also measures the runout of the tire/wheel assembly as it is mounted on the vehicle hub. The on-car runout is preferably measured at two surfaces, and the two surfaces are the same surfaces on the tire/wheel assembly that are measured by the runout measuring device of the off-car balancer. The computer calculates a preferred orientation of the vehicle hub and the tire/wheel assembly and can calculate the magnitude and location of weights for a two-plane balance. The computer preferably includes a user interface to display results to the operator. The on-car runout device and the computer can alternatively be used apart from the off-car balancer to counteract runout in the vehicle hub.
A first advantage of the present invention is to provide an on-car runout device that automatically minimizes the loaded runout of a tire/wheel assembly when mounted on a vehicle hub.
A second advantage of the present invention is to provide an on-car runout device that reduces imbalance in a tire/wheel assembly when mounting the tire/wheel assembly on a vehicle hub.
A third advantage of the present invention is to provide an on-car runout device that can be used in combination with a tire uniformity measuring device to reduce vibration by determining the preferred mounting orientation of a tire/wheel assembly relative to a vehicle hub.
A fourth advantage of the present invention is to provide an on-car runout device integrated into a system that automatically calculates the proper mass and placement of a weight to reduce single-plane imbalance in a tire/wheel assembly when mounted on a vehicle hub.
A fifth advantage of the present invention is to provide a method for determining two-plane imbalances caused by inaccurate mounting of the wheel on a vehicle hub or on an off-car balancer.
A sixth advantage of the present invention is to determine and correct for hubcap induced wheel imbalances.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.